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| United States Patent |
5,783,823
|
|
Mous
, et al.
|
July 21, 1998
|
Apparatus to be used in the field of accelerator mass spectrometry
Abstract
The invention relates to an apparatus to be used in the field of
accelerator mass spectrometry for detection of low concentrations of
isotopes of interest (including, but not limited to chlorine-36) in order
to suppress isobaric interferences from isobars having a lower atomic
number Z (also referred to as "interfering isobars") then said isotopes,
which apparatus comprises means for an at least twofold deceleration of
said isotopes and said isobars by their interaction with matter in order
to create an energy difference between said isotopes and said isobars
based on the Z-dependence of their deceleration.
| Inventors:
|
Mous; Dirk Jozef Willem (Nieuwegein, NL);
Gottdang; Andreas Ulrich (Altenberge, DE)
|
| Assignee:
|
High Voltage Engineering Europe B.V. (NL)
|
| Appl. No.:
|
813988 |
| Filed:
|
March 10, 1997 |
Foreign Application Priority Data
| Current U.S. Class: |
250/281 |
| Intern'l Class: |
H01J 049/26 |
| Field of Search: |
250/281,282
315/500,506,507
313/362.1
|
References Cited
U.S. Patent Documents
| 4037100 | Jul., 1977 | Purser | 250/281.
|
| 4771171 | Sep., 1988 | Snyder et al. | 250/281.
|
| 5237174 | Aug., 1993 | Purser | 250/281.
|
Primary Examiner: Nguyen; Kiet T.
Attorney, Agent or Firm: Jones, Day, Reavis & Pogue
Claims
What is claimed is:
1. Apparatus for use in the field of accelerator mass spectrometry in the
detection of low concentrations of preselected isotopes including, but not
limited to chlorine-36 configured to suppress isobaric interferences from
isobar elements having a lower atomic number Z than said isotopes
comprising means for an at least twofold deceleration of said isotopes and
said isobar elements by their interaction with matter in order to create
an energy difference between said isotopes and said isobar elements based
on the Z-dependence of their deceleration.
2. Apparatus according to claim 1, wherein said deceleration means
comprises a foil acting as interacting matter.
3. Apparatus according to claim 1, wherein said deceleration means
comprises a gas acting as interacting matter.
4. Apparatus according to claim 1, wherein said deceleration means
comprises a plasma acting as interacting matter.
5. Apparatus according to claim 1, comprising means for separating said
isotopes from said isobar elements using an electrostatic and/or magnetic
beam splitter, and a resolving slit.
6. Apparatus according to claim 5 comprising focusing means, including a
quadrupole multiple system.
7. Apparatus according to claim 1, wherein said deceleration means further
comprises a foil being part of a time of flight system measuring the
flight time of said isotopes and said isobar elements between their
passage through the foil and their arrival at a detector.
8. Apparatus according to claim 7, wherein the time of flight system
comprises focusing means located between the foil and the detector.
9. Apparatus according to claim 8, wherein the focusing means are provided
with a plurality of gridded lenses being able to focus the beam of said
isotopes and said isobar elements while keeping the average velocity
thereof at least substantially unchanged regardless of their trajectories
through the time of flight detector.
10. Apparatus according to claim 9, wherein the lenses are alternatively
biased positive, neutral or negative.
11. Apparatus according to claim 1 configured to be used in combination
with a tandem accelerator operating at a terminal voltage of at least
substantially below 5 MV.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an apparatus to be used in the field of
accelerator mass spectrometry (hereinafter also referred to as AMS) for
detection of low concentrations of isotopes of interest (including, but
not limited to chlorine-36) in order to suppress isobaric interferences
from elements having a lower atomic number Z (also referred to as
"isobars") than said isotopes.
2. Description of the Prior Art
Such an apparatus is known from U.S. Pat. No. 5,273,174 (Purser). In the
last decades accelerator mass spectrometry has been recognized as a
technique able to detect extreme low concentrations of isotopes in a
sample. Its unparalleled sensitivity results from the absence of molecular
interferences and the possibility to suppress isobars (U.S. Pat. No.
4,037,100). In AMS negative ions are extracted from the sample under
investigation. After mass analysis, these ions are accelerated in a tandem
accelerator to a terminal maintained at a high positive potential. A
stripper within this high-voltage terminal converts the negative ions to a
positive charge state and induces dissociation of the background
molecules. After further acceleration, mass- and energy-analysis is
performed and the particles of interest are identified by a measurement of
their physical properties. For some elements, AMS rejects the isobar
completely, because the interfering isobar does not form stable negative
ions, which are required for the injection into a tandem accelerator. This
is the case for, among others, .sup.14 C, .sup.26 Al and .sup.129 I and
consequently these elements, which have a widespread field of
applications, can be detected with a sensitivity down to 10.sup.-15
without too many difficulties. In spite of the existence of .sup.10 B-,
.sup.10 Be can also be detected as was shown by Raisbeck et al (see Nucl.
Instr. and Meth. B 5 (1984) 175) by the application of an energy absorbing
foil that preferably stops ions with a higher nuclear charge. On the other
hand, some scientifically important elements like .sup.36 Cl, are well
known for their problematic isobar interferences to be detected at
concentration levels as low as 10.sup.-15. Because sample pre-treatment
and natural abundance reduces the .sup.36 S content in a sample by
approximately 10.sup.-6 and 10.sup.-4 respectively, a further suppression
of 10.sup.5 is needed by the AMS instrument to achieve a background of
10.sup.-15. Contrary to the detection of .sup.10 Be, it is believed by
those skilled in the art that the application of an energy absorbing foil
in the case of .sup.36 Cl does not give the wanted isobar suppression of
10.sup.5 because of the following two reasons: First, the interfering
isobar has a lower atomic number Z than .sup.36 Cl, and as a result a
complete separation of the isobars is, as will be discussed later,
fundamentally impossible. Secondly, the relative Z difference dZ/Z is
considerably smaller, which reduces the energy difference between the
.sup.36 Cl ions and its .sup.36 S isobar for a given foil thickness. In
order to obtain the needed energy difference, the foil must be made so
thick that the transmitted beam becomes too divergent to be transported
properly under normal conditions. As already mentioned, U.S. Pat. No.
5,237,174 proposes a solution to this isobaric interference, wherein use
is made of a so-called "booster accelerator" operating at a terminal
voltage of approximately 4 MV.
A disadvantage of the apparatus according to U.S. Pat. No. 5,237,174 is
that is makes use of a high terminal voltage, whereby the .sup.36 Cl ions
receive enough energy to enable isobaric suppression in a suitable
detector. However, this known apparatus is therefore complicated and, as a
consequence, too expensive to have a wide spread use thereof in medium
sized research institutes and the like.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an apparatus wherein these
disadvantages are obviated and thereto an apparatus of the type mentioned
in the preamble according to the invention is characterized in that it
comprises means for an at least twofold deceleration of said isotopes and
said isobars by their interaction with matter in order to create an energy
difference between said isotopes and said isobars based on the
Z-dependence of their deceleration. In a preferred embodiment of an
apparatus according to the invention said means comprise a (solid) foil
acting as interacting matter. In another preferred embodiment these means
comprise a gas or a plasma acting as interacting matter. Because in two
steps an isobaric suppression of particularly 1000 and 300, respectively,
is achieved, an overall suppression of 10.sup.5 -10.sup.6 is anticipated,
being high enough to allow a detection of .sup.36 Cl with a sensitivity
down to 10.sup.-15. All this is achieved using a terminal voltage equal to
or less than 2.5 MV.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will further be described and elucidated with
reference to the drawing, in which
FIG. 1 is a block diagram showing elements of the present invention;
FIG. 2 is a graph showing an energy spectrum of .sup.36 Cl and .sup.36 S
after passing a foil;
FIG. 3 is a graph showing an angular spectrum of .sup.36 Cl and .sup.36 S
after passing a foil;
FIG. 4 is a schematic part of a focussing means used in a time of flight
system in accordance with the invention, and
FIG. 5 is a graph showing a timing spectrum of .sup.36 Cl and .sup.36 S
after passing a foil.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In a preferred embodiment of an apparatus in accordance with the invention
means are provided for separating said isotopes from said isobars using an
electrostatic and/or magnetic beam splitter, preferably an electrostatic
dipole system, and a resolving slit. Particularly focussing means,
preferably in the form of a quadrupole multiplet system, are used in
combination with the above-mentioned means for separating. As the
invention is characterized by an isobaric suppression in two steps, said
means for separating and (in combination) said means for focussing are
used in the first step, wherein an energy absorbing foil creates an energy
difference between the .sup.36 Cl and .sup.36 S ions, the quadrupole
multiplet system takes care of focussing the beam after passing through
the foil, and afterwards an energy analyzer acting as beam splitter
removes the majority of the .sup.36 S ions from the beam.
In a further preferred embodiment of an apparatus according to the
invention said means for at least twofold deceleration further comprise a
foil being part of a time of flight system measuring the flight time of
said isotopes and said isobars between their passage through the foil and
their arrival at a detector. Particularly these means are integrated in
the second step of the isobaric suppression. The time of flight system
preferably comprises focussing means located between the foil and
detector, which in particular have the form of a plurality of gridded
lenses being able to focus the beam of said isotopes and said isobars
while keeping the average velocity thereof at least substantially
unchanged regardless of their trajectories through the time of flight
system. The lenses are alternatively biased positive, neutral or negative
to obtain the focus effect, while keeping the average velocity at least
substantially unchanged.
The invention also refers to a process to be used with the apparatus in
accordance with the invention.
It is noted that the present invention will be described hereunder with
.sup.36 Cl as an example of the ion of interest and .sup.36 S being the
interfering isobar. Although .sup.36 Cl being a good candidate to benefit
from the present invention, it is not limited thereto, but is generally
applicable to the detection of all ions that suffer from isobaric
interferences having a lower atomic number Z than the ions of interest.
A block diagram showing an AMS system equipped with the present invention
is shown in FIG. 1. Basically, the instrument consists of an accelerator
mass spectrometer system similar to that described in U.S. Pat. No.
5,273,174 (Purser) with the addition of the present invention beyond the
magnetic dipole 8. Negative .sup.36 Cl ions from a suitable ion source 1
are mass analyzed in a magnetic dipole 2 so that only mass 36 particles
pass through the selection aperture (or mass defining aperture) 3.
Following mass analysis, the selected negative ions are directed into the
first acceleration region 4 of a tandem accelerator 5 where they are
accelerated to an energy of 2.5 MeV and directed into a gas dissociation
canal 6. At an energy of 2.5 MeV, a substantial fraction of the ions
leaving the gas dissociation canal 6 will have has six electrons stripped
from the negative chlorine ions (see Wittkower, A. B. and Ryding, G.
"Equilibrium Charge-State Distribution of Heavy Ions (1-14 MeV)", Physical
Review A, 4,226 (1971)) and leave the stripper in a 5+ charge state. These
ions are directed into the second acceleration region 7 where they, with a
terminal voltage of 2.5 MV, receive a further energy gain of 12.5 MeV and
leave the tandem accelerator with a total kinetic energy of 15 MeV. The
next step in the AMS system is the mass analysis of the ions leaving the
accelerator in a magnetic dipole 8, which provides the initial separation
of wanted mass 36 particles from unwanted background ions. Here the most
significant class of background which can pass this magnetic filter
unimpeded are, apart from .sup.36 S.sup.5+, .sup.35 Cl.sup.5+ and .sup.37
Cl.sup.5+ ions which leave the tandem accelerator with the same magnetic
rigidity as 15 MeV .sup.36 Cl.sup.5+. The origin of such backgrounds is
well known by those skilled in the art. It should be mentioned that in the
above description the charge state 5+ is chosen as an example but that the
selection of other charge states might be equally suitable. Up to this
point the geometry of the system is essentially identical to that of the
AMS system as described in the aforementioned patent publication of
Purser. Unique is a particle detector 9 which follows the magnetic dipole
8 (FIG. 1) and which comprehends a suppression stage 10, for the intensity
reduction of isobaric interferences, and a particle identification stage
11 which discriminates the remaining .sup.36 S ions from the .sup.36 Cl
ions. Both stages will be described in the following paragraphs.
Suppression stage
The .sup.36 Cl ions enter the suppression stage through an entrance foil 12
where they are decelerated according to the well known Bethe Bloch
relationship:
##EQU1##
in which dE/dx is the energy loss of the ions per unit length, Z is the
atomic number of the ions and v being the velocity of the ions.
Consequently, energy discrimination between .sup.36 Cl and .sup.36 S
results from the difference in their Z-value. However, apart from energy
discrimination, passage through the foil creates, as known to those
skilled in the art, unwanted energy straggling and small angular
scattering. FIGS. 2 and 3 show the energy and angular distribution of the
.sup.36 Cl and .sup.36 S ions, after the passage through a 2 micrometer
thick mylar entrance foil. In these simulations the energy of the ions
before the entrance foil was taken to be 15 MeV. After passage through the
entrance foil the charge state 8+ (other charge states might be suitable
as well) is selected for further transport and the ions pass a special
quadrupole multiplet 13, which serves for a proper focussing of the
extreme divergent beam on the energy resolving slit 15. Following the
quadrupole multiplet an electrostatic dipole (energy analyzer) 14 bends
the ions according to their energy. Because of their slightly different
energy loss in the foil, .sup.36 S ions have a slightly higher energy than
the .sup.36 Cl ions and as a consequence they can be stopped at the energy
resolving slit 15. However, as can be seen from FIG. 2, part of the
.sup.36 S ions will pass the resolving slit system 15 because the low
energy tail of the .sup.36 S ions which extent into the .sup.36 Cl energy
window which is determined by the width of the energy resolving slit 15.
It is anticipated that the proposed suppression stage will reduce the
.sup.36 S intensity by a factor of approximately 1000.
The particle identification stage
The second part of the particle detector directly follows the suppression
stage and is formed by a unique particle identification stage 11 which
features a velocity measurement of the ions by measuring the flight time
of the ions between the moment of passage of the ions through a start foil
and the moment of arrival at a stop detector. This technique is called
time of flight (TOF) and is well understood by those skilled in the art.
The stage consist of the TOF start foil 16, a focussing structure 17
comprising multiple gridded lens elements, and the TOF stop detector 18.
Apart from the TOF measurement, the particle identification stage is
unique in its optical acceptance of extreme divergent beams, which result
from the small angular scattering in the TOF start foil 16. Moreover, the
stage keeps the average velocity of the ions at least substantially
unchanged, regardless of their trajectories through the focussing
structure 17. A detailed description of the particle identification stage
is given below.
When the ions are transmitted through the TOF start foil 16 a number of
physical processes occur:
1) the .sup.36 Cl and .sup.36 S ions are decelerated according to the
relationship ›1!. As a result of their different Z-values, the .sup.36 Cl
and .sup.36 S ions leave the foil with different velocities, which makes
the discrimination of these ions possible in a TOF measurement.
2) A continuum of different charge states of the ions is created which
result from charge changing processes with the foil.
3) A pulse of secondary electrons is created when the ions pass the foil.
This pulse is detected and used as a starting pulse for the TOF
measurement, as is well known by those skilled in the art.
4) Small angular scattering in the foil creates an extreme divergent beam.
The extreme divergence of the beam results from the foil thickness that is
substantially thicker than foils that are normally applied in TOF
experiments. In the present invention such a foil thickness is needed to
obtain the wanted velocity difference as described in 1).
After passage through the foil, some means of focussing is needed to direct
the extreme divergent beam onto the TOF stop detector 18. In the present
invention this is done by the focussing structure 17 that is located
between the TOF start foil 16 and the TOF stop detector 18. The focussing
structure 17 should be able to focus the extreme divergent beam that
consists of various charge states into a beam spot that is sufficiently
small to be detected at the TOF stop detector 18. In addition to this, it
should keep the average velocity of the ions undisturbed, regardless of
their trajectories through the focussing structure 17. If the last
requirement is not fulfilled, there will be timing uncertainty of ions
with equal initial velocities and as a consequence the time resolution of
the TOF measurement is affected. As known to those skilled in the art,
electrostatic focussing elements such as quadrupoles and einzellenses
inherently decelerate and/or accelerate ions within the lens structure as
a function of their distance to the optical axis. This disturbs the
average velocity of the ions transported and affects the flight time in
the case of a TOF measurement. In the present invention, focussing action
is created in the focussing structure 17 which consist of a number of
gridded lenses and which keeps the average velocity of the ions at least
substantially unchanged, regardless their trajectories through the
focussing structure. FIG. 4 shows a drawing of one section of the
focussing structure. It is anticipated however, that in the present
invention several such sections will be placed adjacent one another to
form the total focussing structure. The section consists of four grids 1
(numbered 1 through 4) which are essentially transparent for ions and
three cylindrical electrodes 2, some of which are biased positive, neutral
or negative. Some of the equipotential lines 3 and the direction of the
electrostatic forces 4 that are experienced by the transported ions are
shown as well. Those skilled in the art readily recognize the overall
focussing power of such one section because the radial component of the
electrostatic force always points to the optical axis 5. More importantly
they will also recognize that such a section creates an overall
deceleration-acceleration-acceleration-deceleration action on the
transported ions as is indicated in the figure. By a proper adjustment of
the distances between the grids, the velocity los of the ions between the
first two grids can be compensated by the velocity gain between the grids
numbers 2 through 4, and consequently a structure can be created which
keeps the average velocity of the ions essentially unchanged, regardless
their trajectories through the focussing structure. The timing performance
of a focussing structure, that comprises 3 individual sections which is
capable to deliver the required focussing power and which has a total
length of 1.8 meters, has been simulated for the transmission of .sup.36
Cl and .sup.36 S ions having an energy of approx. 4 MeV. The result of
these simulations, in which the influence of the different charge states
of the ions and the small angular scattering were taken into account, is
shown in FIG. 5. It can be seen that a timing uncertainty of approximately
1 nanosecond is anticipated, which is low enough to be able to
discriminate .sup.36 Cl and .sup.36 S in the TOF measurement. It is
anticipated in the particle identification stage will further suppress the
isobaric .sup.36 S interference by a factor of approximately 300.
From the above paragraphs it can be seen that a particle detector
consisting of the two stages placed adjacent one another as described
above is able to suppress .sup.36 S isobars in a .sup.36 Cl analysis by a
factor 10.sup.5 -10.sup.6. Such a suppression is high enough to allow for
the detection of .sup.36 Cl ions in an AMS analysis with a sensitivity of
approx. 10.sup.-15.
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